There have been a number of studies supporting the efficacy of spinal manipulation, the signature therapy of chiropractic care, for cervicogenic headache (CGH). Yet, there have been no large trials to identify the number of treatment visits to optimize effectiveness and cost-effectiveness of care. Our pilot studies were the first trials to investigate a dose-response relationship between the number of treatments with spinal manipulation (dose) and relief of CGH. The purpose of our study is to determine dose-response and cost-effectiveness of spinal manipulation provided by chiropractors for CGH. To accomplish this goal, Western States Chiropractic College and Northwestern Health Sciences University will conduct a 5-year multicenter randomized trial. The study protocol is designed to control attention and the laying on of hands. A total of 256 participants will be randomized to 4 treatment groups (n = 64/group). All participants will attend 18 sessions with a chiropractor: 3 visits per week for 6 weeks. All participants will receive treatment at each visit. The dose levels will be 0, 6, 12, or 18 sessions for manipulation or a light massage alternative. The primary specific aim is to determine the effect of the number of spinal manipulation treatments provided by a chiropractor for the care of chronic CGH on clinical outcomes and objective physical measures. The primary outcomes will be CGH pain and number. Outcomes will be evaluated at baseline, 6, 12, 18, 24, 39, and 52 weeks. The secondary specific aim is to determine the cost-effectiveness and cost-utility of the number of spinal manipulation treatments for the care of chronic CGH. The tertiary specific aim is to assess effects of expectations on outcomes using quantitative and qualitative methods. The study will be significant in that it will be the first fully powered trial to investigate optimal dose and cost- effectiveness of spinal manipulation for the care of headache, an important concern for the health of the public. It will provide high quality scientific evidence that will inform evidence-based treatment protocols for clinical practice, third-party payer reimbursement, and schedules of chiropractic care used in future trials on chronic headache. The study will be innovative in controlling/monitoring the doctor-patient encounter, using a protocol developed for randomized trials where participant/provider blinding is not feasible. A qualitative component will also add insight into the relationship of patient expectations with outcomes. The long-term goal is to conduct studies based on optimal care including a phase III trial of efficacy and comparative effectiveness.